1. Field of the Invention
The present invention relates to means for separating an image recording medium from an image carrier after a transfer of toner images from the image carrier to the image recording medium in an electrophotographic recording apparatus.
2. Description of the Related Art
Widespread use of office automation systems requires high quality output apparatuses in which ordinary paper can be used. For this purpose, electrophotographic recording apparatuses such as copying machines and facsimile apparatuses have been realized, and are disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 53-104246 and No. 2-135385. The electophotographic recording apparatuses include an image carrier such as a light-sensitive drum carrying toner images developed thereon, conveying rollers for conveying an image recording medium such as paper to the image carrier in a predetermined direction for transferring the toner images from the image carrier to the image recording medium, and a transfer unit arranged at a specific distance from the image carrier and having a charging wire electrode causing a corona discharge for electrically charging the image recording medium. The transfer unit typically charges the image recording medium to a plus potential to which minus charged toner on the image carrier is attracted.
In these electrophotographic recording apparatuses, the image recording medium is brought into contact with the image carrier at a certain contact portion for transferring the toner images from the image carrier to the image recording medium and then the image recording medium is disengaged from the image carrier for directing the image recording medium to a fixing process. However, the image recording medium is apt to be attracted to the image carrier after the toner images are transferred from the image carrier to the image recording medium. Therefore, it is necessary to separate the image recording medium from the image carrier so that the transferred images are not disturbed, to obtain high quality products jamming of the image recording medium does not occur.
To separate the image recording medium from the image carrier, it is known to provide a DC separating structure comprising a row of deelectrification needles on the downstream side of the transfer unit as viewed from the paper conveying direction, the needles being connected to a minus pole of a power source to pull the plus charged image recording medium and thereby to separate the image recording medium from the image carrier. However, the DC separating structure requires a large expenditure for arranging, for example, a plurality of deelectrification needles.
A curvature separating structure is also known in which the radius of curvature of the image carrier (light-sensitive drum) is made smaller and the image recording medium departs from the image carrier at a relatively large angle between the image recording medium and the tangent line to a certain point of the image carrier. In this case, however, the available surface area of the image carrier is small and the limited surface is used excessively, thereby shortening the life of the image carrier. Further, a face-up separating structure is also known in which the image recording medium is conveyed under the image carrier thereby to engage same from below and thus the separation of the image recording medium from the image carrier is assisted by the weight of the image recording medium. In this case, however, there are restrictions in designing the face-up separating structure rather than the face-down separating structure.
The above described Japanese Unexamined Patent Publication (Kokai) No. 53-104246 and No. 2-135385 also disclose means for separating the image recording medium from the image carrier. In Kokai No. 53-104246, the means for separating the image recording medium from the image carrier comprises a conductive guide roller and a dielectric material covering the conductive guide roller, with the conductive guide roller being connected to the ground or a minus pole of a power source. In Kokai No. 2-135385 , the means for separating the image recording medium from the image carrier comprises a conductive guide plate and a dielectric layer sandwiching the conductive guide plate, with the conductive guide plate being connected to a minus pole of a power source. It is said that if the conductive guide roller or plate directly contacts the image recording medium, the conductive guide roller or plate may disturb the toner images on the image recording medium and so the dielectric layer is provided to prevent the conductive guide roller or plate from directly contacting the image recording medium. In these publications, the means for separating the image recording medium from the image carrier is provided as a special component in addition to the transfer unit. Therefore, these means increase the manufacturing cost of the apparatus.